I happened upon a bit of info that I hadn't seen documented, so I wanted to pass it along.
I had several users getting Error 55501, and were unable to open or otherwise download files. I couldnt figure out why initially, and saw the post
I realized he project in question was in a new storage area. Even stranger, only external users were affected - it worked in the local office (the same office where the Integration Server is located).
The fix: in PW Administrator, when looking at the properties on the storage areas, the "Computer Name or IP address:" field, the Integration server name was entered as: "servername". It did not have the fully qualified domain name (FQDN). I thus opted to re-name it using the FQDN thus: "servername.company.com" and not just "servername". That turned the trick and eliminated Error 55501 for external users.
Hope this is of help.